Pipelines are often buried to reduce the likelihood of accidental contact with ships' anchors or fish trawling apparatus. Further risks arise for oil or gas pipelines in areas, such as the southern North Sea, where the water is shallow and tidal currents can scour silt from beneath pipelines leaving them unsupported. Unsupported pipelines are liable to fracture and thereby cause significant environmental damage. When a pipeline has been installed, a contracted depth of burial normally has to be demonstrated to have been achieved. In addition, it is necessary to survey the entire length of an installed pipeline periodically to determine whether or not the pipeline remains buried. Determining if the buried depth of a pipeline has changed or if a pipeline has been displaced laterally may also be helpful.
A sub-sea survey of an installed pipeline normally provides the key parameters of a record of the pipeline track and Depth of Burial (DOB) of the pipeline together with a video of the seabed. Sub-sea surveys are often carried out by a Remotely Operated Vehicle (ROV) carrying pipe tracking and DOB measurement apparatus.
Known approaches to pipe measurement include the magnetometer, which measures local changes in the earth's magnetic flux caused by a target pipe, and passive systems, in which an electrical tone is applied to a pipe to be tracked and the applied electrical tone is detected by a sensing device. Pulse induction apparatus provides a third known approach to pipe detection in which a pulse of changing magnetic flux emitted by a transmitting device is used to induce a changing eddy current in a conductive target pipe. The induced eddy current in turn induces a changing magnetic flux that is detected by a receiving device. The transmitting device and the receiving device normally comprise at least one flat coil of wire.
A limitation of known pulse induction apparatus arises from the inverse cube law of induced fields. The field produced by a flat current loop falls by a denominator term comprising [a2+b2]3/2, where ‘a’ is the diameter of the coil and ‘b’ is the distance along the axis from the plane of the coil. The numerator term also comprises a2. Thus, as ‘b’ increases the field falls by 1/b3. As the target eddy current functions as a second flat current loop, the attenuation from transmitting device to target pipe to receiving device increases as the sixth power of distance. This means that a signal voltage is divided by sixty-four for each doubling of distance. Therefore, transmitted pulses are significantly greater than received pulses in known pulse induction apparatus. Typically, the transmit pulse can be as much as 106 greater than the received pulse in a practical pulse induction apparatus.
The difference in magnitude between a transmitted pulse and a received pulse gives rise to another problem. More specifically, most of the high rate of change eddy current in the target pipe overlaps in time with the transmitted pulse. Thus, direct inductive and capacitive coupling between the transmitting device and the receiving device normally overloads the receiving device. A known approach to this problem is to turn the receiving device off when the transmitting device is operative to transmit a pulse. In addition, the length of time that the receiving device is turned off is extended by a predetermined amount to take account of transient response limitations of electronic circuitry of the receiving apparatus. U.S. Pat. No. 3,315,155 (to Claus Colani) describes such a pulse induction apparatus, in which the receiving device makes use of only the decaying tail of a pulse received from a target pipeline.
It is an object for the present invention to provide an improved apparatus that is operative on the basis of the above described eddy current induction approach.